#define _CRT_SECURE_NO_WARNINGS 1

#include <iostream>
using namespace std;

class Sum
{
public:
	Sum()
	{
		_sum += _k;
		_k++;
	}

	static int Get_sum()
	{
		return _sum;
	}

private:
	static int _k;
	static int _sum;
};

int Sum::_k = 1;
int Sum::_sum = 0;

int main()
{
	int n = 0;
	cin >> n;
	Sum* ps = new Sum[n];

	cout << Sum::Get_sum() << endl;

	return 0;
}